The present invention relates to silicon-on-insulator (SOI) substrates for use in the semiconductor industry for fabricating integrated circuits (ICs), and more particularly to a separation by implantation of oxygen (SIMOX) process in which the buried oxide region is highly uniform and has thermal oxide-like qualities.
SIMOX is a technique that is employed in fabricating SOI substrates which can be used in the manufacturing of ICs. SIMOX typically involves using high-energy ions to implant a large dose of oxygen ions beneath the surface of a bulk Si wafer. Upon high-temperature annealing, the implanted oxygen forms a continuous buried oxide (BOX) region which electrically isolates the Si at the surface (i.e., the Si superficial layer). Typically, prior art SIMOX processes have been used to fabricate SOIs with a superficial Si layer and a BOX thickness of several thousand angstroms.
Conventional SIMOX processes include one of the following methods:
(i) a high-dose oxygen implantation (greater than about 4E17 cmxe2x88x922) step followed by annealing at temperatures of greater than about 1300xc2x0 C. in an inert ambient such as Ar or N2 that contains less than about 5% oxygen.
(ii) a high-dose oxygen implantation (greater than about 4E17 cmxe2x88x922) step followed by annealing at temperatures of greater than about 1300xc2x0 C. in an inert ambient such as Ar or N2 that contains a much higher content of oxygen than in method (i). Typically, the oxygen content used in the annealing step in this method is between 30-40% which is employed to promote internal thermal oxidation.
(iii) Combining a base oxygen implant of greater than about 1E17 cmxe2x88x922 and a room temperature implant (typically greater than about 1E15 cmxe2x88x922) followed by annealing in an inert ambient that contains oxygen in a concentration range of from about 8 to about 40%. The limit on oxygen content is imposed because of the range of an oxygen beam in the Si substrate using a conventional implanter operating at a nominal energy of about 200 keV; Si thickness is less than about 4500 xc3x85. Since greater than 4-5 hours annealing is typically required at temperatures greater than 1320xc2x0 C. to achieve device quality SIMOX with desired Si thickness, the inert ambient is limited to less than 40%.
The conventional SIMOX processes mentioned above each suffer from the same problem in that the prior art SIMOX processes are incapable of forming a BOX region which is composed substantially of a thermal oxide layer, i.e., an oxide that is formed by thermal oxidation. Instead, the prior art SIMOX processes mentioned above produce BOX regions that are composed mainly of an oxide layer that is formed by implanted oxygen ions. A thermal oxide layer is preferred in the semiconductor industry over oxide layers that are formed by implanted oxygen ions because thermal oxides have fewer defects, have a sharper Si/BOX interface and are of higher quality (in terms of both structural and electrical quality) as compared with an oxide layer formed by implanted oxygen ions.
In view of the drawbacks mentioned above with prior art SIMOX processes, there is a continued need for developing a new and improved SIMOX process that is capable of forming a BOX region which is composed substantially of a thermal oxide layer.
One object of the present invention is to provide a method of forming a BOX region in a Si-containing substrate in which the BOX region is composed substantially of a thermal oxide layer.
A further object of the present invention is to provide a method of forming a BOX region in a Si-containing substrate that exhibits improved structural qualities as well as electrical qualities as compared to BOX regions that are formed using conventional SIMOX processes.
A yet further object of the present invention is to provide a method of forming a BOX region in a Si-containing substrate in which substantially little or no divot defects are formed in the final SOI substrate.
A still further object of the present invention is to provide a method of forming a BOX region which has a substantially uniform interface with the superficial Si-containing layer that is formed on top of the BOX region.
These and other objects and advantages are achieved in the present invention by utilizing a method which ensures that there is sufficient Si thickness present during the entire SIMOX process such that internal thermal oxide growth caused during the annealing step is enhanced. By enhancing the internal thermal oxide growth, a BOX region containing a greater content of thermal oxide as compared to oxide formed by implanted oxygen ions is formed.
Specifically, the present invention provides a SIMOX process which comprises the steps of:
(a) implanting oxygen ions into a surface of a Si-containing substrate, said Si-containing substrate including a sufficient Si thickness to allow for subsequent formation of a buried oxide region in the Si-containing substrate which has a greater content of thermally grown oxide as compared to oxide formed by implanted oxygen ions; and
(b) annealing the Si-containing substrate containing said implanted oxygen ions under conditions that are effective in forming said buried oxide region containing said greater content of thermally grown oxide as composed to oxide grown by said implanted oxygen ions.
It is noted that the term xe2x80x9cSi-containing substratexe2x80x9d as used herein denotes semiconductor substrates such as Si, SiGe, SiC, SiGeC, Si/SiC, Si/SiGe as well as patterned or unpatterened preformed silicon-on-insulators (SOIs) which include a single or multiple buried oxide regions formed therein.
Because the method of the present invention forms a buried oxide region having a greater content of thermal oxide as compared to oxide formed by implanted oxygen, the inventive method is referred to herein as the ultimate SIMOX process.
In accordance with a first embodiment of the present invention, sufficient Si thickness is maintained during steps (a) and (b) by forming a Si layer atop of the Si-containing substrate. In this embodiment of the present invention, the Si layer is formed atop the Si-containing substrate after implanting the wafer with oxygen ions, but prior to annealing. The implant in this embodiment is carried out using a low-dose oxygen implant step and annealing is carried out at high-temperatures in an ambient that includes from about 0.1 to about 100% oxygen and from about 0 to about 99.9% of an inert gas.
In accordance with a second embodiment of the present invention, the sufficient Si thickness is maintained by conducting a high-energy, high-dose oxygen implant to ensure that the oxygen ions are implanted a sufficient distance from the surface of the Si-containing substrate. Annealing is carried out at high-temperatures in an ambient that includes from about 0.1 to about 100% oxygen and from about 0 to about 99.9% of an inert gas.
In a third embodiment of the present invention, the buried oxide region having a greater content of thermal oxide as compared to oxide formed by ion implantation is carried out using a high-energy, low-dose oxygen implant step. Using a high-energy, low-dose implant process provides less implanted oxygen therefore allotting for a greater growth of thermal oxide. In this embodiment, annealing is carried out at high-temperatures in an inert gas atmosphere that includes from about 0.1 to about 100% oxygen and from about 0 to about 99.9% of an inert gas.
Notwithstanding which of these embodiments is employed, the implanting of oxygen ions in step (a) may be carried out using a single ion implantation step or multiple ion implantation steps such as a base oxygen implant followed by a low-temperature ion implant may be employed.
Another aspect of the present invention relates to SOI substrates that are formed utilizing the above-mentioned ultimate SIMOX process. Specifically, the inventive SOI substrate comprises:
a buried oxide region that is sandwiched between a top superficial Si-containing layer and a bottom Si-containing layer, wherein said buried oxide region has a greater content of thermally grown oxide as compared to oxide formed by implanted oxygen ions and said buried oxide region includes an interface that is uniform with said top superficial Si-containing layer.
Because of the increased content of thermally grown oxide, the buried oxide regions of the present invention exhibit high structural as well as electrical qualities as compared to buried oxide regions formed using conventional ion implantation.
The term xe2x80x9chigh structural qualityxe2x80x9d is used herein to denote a SOI substrate which has little or no etch pitch density (less than about 1xc3x97105 cm2); little or no top or bottom Si/buried oxide interface roughness (interface roughness of less than about 100 xc3x85 as measured by TEM (transmission electron microscopy)); a low HF-defect density (less than about 1 cm2); and a low surface roughness (of about 6 xc3x85 root mean square).
The term xe2x80x9chigh electrical qualityxe2x80x9d is used herein to denote a structure wherein the buried oxide breakdown field is high (greater than about 6 megavolts per cm); the buried oxide minibreakdown voltage is high (greater than about 50 volts); the buried oxide leakage at a given voltage is low (less than about 1 nanoAmp); and the buried oxide density is low (less than about 1 cmxe2x88x922).